1. Field of the Invention
The present invention relates to a mobile communication system, a radio network controller, a base station and a communication method.
2. Description of the Related Art
Conventionally, broadcast communication and multicast communication have been used. In broadcast communication, abase station simultaneously transmits, to an unspecified number of mobile stations located in a radio area covered by the base station, data common to the unspecified number of mobile stations. In multicast communication, a base station transmits, to mobile stations belonging to a specific group, data common only to the mobile stations belonging to the specific group (see, for example, 3rd Generation Partnership Project Technical Specification Group Terminals, “23.041 Technical realization of Cell Broadcast Service (CBS)”, October 2000; and 3rd Generation Partnership Project Technical Specification Group Radio Access Network, “25.324 Broadcast/Multicast Control BMC”, December 2000).
In one form of multicast communication, the data to be transmitted by multicast communication is layered, i.e., divided into plural layers, and then the layered data is transmitted (see, for example, Samsung, “MBMS-000033 Scalable MBMS (slides)”, 3rd Generation Partnership Project MBMS Workshop, May 2002; and Lucent Technology, “3GPP TSG-RAN, R2-022110 MBMS power usage”, August 2002).
In conventional multicast communication, however, the condition of radio resources for respective radio areas covered by base stations, which transmit data is not taken into consideration at all. As a result, when the condition of radio resources for respective radio areas is different, some of the base stations may run short of radio resources and fail to transmit data. On the other hand, some of the other base stations have surplus radio resources, which may be wasted. If there are few available radio resources, multicast communication may apply pressure on the traffic of data transmission other than the multicast communication.